The rate of increase in the world's population peaked in 1970 and has begun to decline, scientist monitoring the subject announced yesterday.

The world's population, now at 4.1 billion, was rising at 1.9 percent per year in 1970, but the rate had fallen to 1.7 percent by last year, said Nick Eberstadt for Harvard University's Center for Population Studies.

But Eberstadt and other population experts cautioned during a symposium at the American Association for the Advancement of Science convention here that it is too early to be sure the shift is permanent.

It is also too early to be sure what caused it. The population experts grudgingly agreed that at least two-thirds of the decline seemed to be associated with, if not necessarily caused by, rising standards of living in underdeveloped countries where the drop was most dramatic. Another large portion of the decline seems to be associated with intensive family planning programs, they said.

"Nearly all the countries with 20 percent or more decline in birth rates [between 1965 and 1975] have had a strong or moderately strong family program," said Parker Mauldin, senior fellow at the New York-based Population Council.

Those include China, which claims 24 percent reduction in its birth rate over that decade, South Korea (32 percent), Thailand (23 percent), Columbia (25 percent), North Vietnam (23 percent), Taiwan (30 percent) and Chile (29 percent), among the larger nations.

Developing countries as a whole, with half the world's population, have gone from 42 births per 1000 persons per year in 1970 (excluding China) to 36 per 1,000 in 1977, Eberstadt said. The U.S. rate is 14 per 1,000.

The figure indicate that the world's population will double in 41 years instead of the 36 years it would have taken at the higher rate, Eberstadt said.

"The population of the earth is certain to continue growing well into the next century The momentum is just tremendous, and there's no way we could conceivably stop, except for massive catastrophe, short of 8 to 10 billion persons," Mauldin said.

Whole areas of the earth have not yet begun to reduce their birth rate, he said: Bangladesh, Pakistan, and most African nations. The decline has been fastest in Asia, at 17 percent between 1965 and 1975, excluding China, and slowest among blacks and Moslem countries, he said.

Although China undeniably leads the world in its determination to control population growth, some of its statistics are suspect for that very reason, said longtime China watcher John Aird of the Commerce Department. Most educated guesses put China's population now at a minimum of 914 million, with a high probability that it is between 930 million and 960 million, he said, far more than the 800 million figure used in past population reports.

China says its birth rate has dropped from 34 per, 1,000 persons per year in 1965 to 26 per 1,000 in 1975. "There is lots of commotion [for publicity purposes] of model units and some fabrication of figures in the provinces as government pressure for [birth control] results intensifies," Aird said. "Overcoming the traditional opposition to late marriage and birth control is a slow, difficult task."

Robert Repetto of Harvard University's school of Public Health told the all'day symposium that distributors of wealth among poor people lowered their birth rate. "The more the large fraction of the population is included in the modernization process, the more fertility declines," he said. "This lays to rest the alleged tradeoff between growth and equity . . . Efforts to improve the welfare of the poor will contribute to fewer births, more rapid accumulation of capital."

Repetto cited studies where birth rates fell in nations where income gaps were reduced: China, Taiwan, South Korean, Sri Lanka and Malaysia. Little drop occured in Brazil and the Philippines, where the rich-poor gap remains wide although overall economic growth has skyrocketed, he said.

High birth rates arise from generally national rational causes, Moni Nag of the Population Council said. Parents have more children if many are likely to die in childhood. An uneducated, undernourished child will still earn enough to pay for himself or herself by the age of 15 and will provide for the parents thereafter. "Not until the costs for children rise and their returns drop should one expect birth rates in a nation to come down," Eberstadt said.

Other speakers at the conference emphasized the role of improved nutrition and public health services in improving childbearing capacity and therefore making contraceptive distribution more important.

Wililam Peterson of Ohio State University attacked government and international agency efforts to promote birth control, saying interference with native cultures and heavy-handed approaches had occasionally caused an increase of a decrease in the birth rate.